P0537

P0537 sets when the A/C evaporator temperature sensor (a thermistor mounted in or on the evaporator core inside the HVAC case) reports a voltage below the calibrated low threshold, which the HVAC controller interprets as an unrealistically cold evaporator or a shorted sensor. Because the controller uses this sensor to cycle the compressor and prevent evaporator freeze-up, a low-voltage fault can either disable the A/C entirely or, worse, allow the evaporator to actually freeze if the controller defaults to assuming cold. Cheapest-first: unplug the sensor connector (usually accessible behind the glove box or under the dash on the passenger side) and measure resistance across the thermistor at ambient temperature, comparing to spec (typically 8 to 15 kohms at 70F). If the resistance is shorted or way off, replace the sensor; if the resistance is correct, the fault is in the wiring or the HVAC controller's analog input. Caveat: many of these sensors are physically inserted into the evaporator fins and require partial dash or HVAC case disassembly.

Vehicle-specific patterns: 2006-2012 GM full-size trucks and SUVs (Silverado, Tahoe, Yukon, Suburban) see the evaporator temp sensor fail from condensate corroding the pigtail, often combined with a blend door actuator failure that produces the same no-cold-air complaint. 2007-2013 Ford F-150 and Expedition see the sensor itself fail and frequently get misdiagnosed as a low-charge complaint. 2008-2014 Dodge Ram and Durango have a known issue where the sensor wire chafes against the HVAC case. 2010-2015 Hyundai Sonata, Santa Fe, and Kia Sorento fail at the sensor connector from cabin humidity. Estimated repair: $90 to $480.